Field of the Invention
This invention relates to improvements in machine tool control systems and more particularly, but not way of limitation, to a metal working machine control system for moving a cutting tool through a metal workpiece whereby the cutting tool maintains a constant thrust against the workpiece.
In the past it has been recognized that in the cutting of metal, especially for aerospace applications, that improvements in such cutting methods was essential. While variations in material are unimportant when cutting soft materials with hard tools, such variations become very important when cutting hard material with only slightly harder tools. With the advent of increasingly hard to machine materials, which materials would include heat resistant materials that are low in Rockwell number as well as hard materials, great difficulties have been encountered in providing cutting tool control systems that adequately and economically meet the needs for cutting hard materials. While there have been the traditional problems of attempting to eliminate the danger of drill breakage and increasing production rate by decreasing the amount of metal cutting time as well as avoiding effects on the metal by the cutting metal procedure itself such as generating excessive heat in producing a hole, there have been additional problems encountered.
As the cost of energy, among other manufacturing costs, has gone up the cost of raw materials used in aerospace applications has reached unprecedented heights thereby making it imperative that the amount of scrap be reduced to the minimum in the manufacturing process. While the cost of material has gone up, the cost of direct labor has also risen to previously unknown highs thereby making it doubly important that the production rate be increased to the greatest extent possible. Unfortunately, while these needs have become increasingly important, the availability of skilled workers for such manufacturing processes has diminished.
It was possible in the past to conduct some metal machining processes, without extensive additional machine controls, solely by using skilled machinists who could, by reason of long experience, provide a touch or feel for manually regulating the processes. However, even with such experienced personnel, the touch of such personnel would diminish over the course of the day as a person would become tired and the manufacturing efficiency of the process would decrease. Since it is becoming increasingly necessary to employ relatively inexperienced personnel in conducting certain metal working processes, it is essential that control systems be provided that would provide for the metal machining process to be conducted at optimum efficiency not only from the point of view of time required for the process but also for the process to be conducted in the most economical manner to preclude such effects such as tool breakage, undue regrinding of the tool, and heat treating of the material by the inherent aspects of the metal machining process. It is believed that the present metal machining control system achieves more of the required standards and avoids more of the difficulties than any metal machining control system heretofore known in the art.
One known process for controlling the operation of the machine tool entails measuring the working torque applied by a cutting tool to the workpiece, the power applied to the tool and the deflection to which the tool is subjected during working. In such a process the quantities which are controlled are the cutting speed and the advance speed of the tool. Control quantities are tied to quantities measured by rather complicated predetermined equations. Control of the cutting feed and of the feed speed of the tool is affected by an analog computer which solves said equations. This process suffers from fundamental disadvantage of using specially programmed measuring instruments that are both very complex and costly.
Processes are also known for the control of production machining, by means of machine tools, of a series of workpieces in which the temperature of the cutting region is measured and the state of the cutting tool is also monitored constantly, these two parameters being utilized in the automatic control of the cutting speed of the tool. The fundamental disadvantage of such processes lies in the fact that the measurement of the temperature in the cutting region is particularly complicated and difficult. In fact, this temperature is influenced by the mass flow of coolant, the form of cutting produced and by variations in the conditions of contact between the cutting tool and the workpiece, which in turn depend on the type of tool and which vary with each sharpening of the tool. The temperature in the cutting region therefore undergoes considerable variations even over very short periods, and in practice the temperature measurements will be limited to the mean temperature over a period and the mean temperature of an area of the workpiece of excessive dimensions.
In another metal working machine, a number of different conditions relating to the cutting tool are sensed and acted upon to maintain the spindle torque substantially equal to a command value. The conditions which are sensed include the feed velocity, the feed position, the feed thrust, the spindle velocity and the spindle torque. The feed of the cutting tool and the rotational velocity of the spindle are adjusted to adjust the spindle torque substantially equal to a predetermined value. Another requirement of this process requires the determination of tool geometry and the special grinding of such tools to permit this process to operate successfully. The optimum values of feed, velocity, spindle torque and spindle velocity as related to the cutting edge and contact for a specfic tool and material are to be determined by analysis and testing. Quite obviously, it requires a very complicated control system in order to sense a multiplicity of values, to process them, and to then control the torque of the machine tool. Additionally, it takes the skills of very experienced operators, not only used to operating machine tools but also to utilizing electronic control systems, to control such machine tools. Quite clearly, such operators are expensive and not readily available.
A related system was a machine control system utilizing specially wound DC motors. The specially wound DC motors were settable to have particular speed-torque droop characteristics. This speed droop characteristic was used to regulate the torque of the machine as the feed of the machine tool was also regulated. Since the thrust of the tool against the workpiece was interdependent with the torque of the tool rotation, the torque of the tool rotation drive was maintained substantially equal to a predetermined ratio with the thrust. This machine tool control system does require the use of specially wound DC motors and relies upon the operator being skilled at setting the motor characteristics and the ratio of thrust to torque to be able to operate the system satisfactorily. Thus, the need for special equipment and the need for specially trained skilled operators does not render this process to be advantageous for today's conditions.